1. Field of Invention
The present invention relates to a direct voltage converter. More particularly, the present invention relates to a timing control method of a direct voltage/direct voltage converter.
2. Description of Related Art
FIG. 5 is a circuit diagram of a conventional voltage mode pulse width modulation (PWM) direct voltage/direct voltage converter. The voltage mode PWM direct voltage/direct converter comprises a switch-rectifying circuit 500, a first resistor 514, a second resistor 516, a differential amplifier 524, a comparator 520, a triangular wave generator 522 and a driving device 518. As show in FIG. 5, the switch-rectifying circuit 500 has a switch for receiving an input voltage. Thereafter, the input voltage is shaped and filtered to produce an output voltage. The differential amplifier 524 amplifies the error voltage between the input voltage and a reference voltage. The comparator 520 compares the amplified error voltage with the output from the triangular wave generator 522 and outputs a driving signal to the driving device 518. Finally, the driving device 518 outputs a turning-on or switching-off signal to a switching device 510 inside the switch-rectifying circuit 500. Due to the deployment of an analogue modulation method to modulate the pulse width, this type of converter must incorporate analogue circuits including the triangular generator 522 and the differential amplifier 524. With these analogue devices, the converter is very sensitive to any process variation and the output voltage may deviate slightly from the target voltage.
FIG. 6 is a circuit diagram of a conventional current mode pulse width modulation (PWM) direct voltage/direct voltage converter. The current mode PWM direct voltage/direct voltage converter comprises a switch-rectifying circuit 500, a current monitoring circuit 512, a first resistor 514, a second resistor 516, a differential amplifier 524, a comparator 622, a flip-flop 618 and a pulse generator 620. As shown in FIG. 6, the switch-rectifying circuit 500 has a switch for receiving an input voltage. Thereafter, the input voltage is shaped and filtered to produce an output voltage. The differential amplifier 524 amplifies the error voltage between the input voltage and a reference voltage. The comparator 622 compares the amplified error voltage with the output from the current monitoring circuit 512 and outputs a signal to the flip-flop 618. According to the pulses received from the pulse generator 620 and the signal received from the comparator 622, the flip-flop 618 outputs a turning-on or a switching-off signal to a switching device 510 inside the switch-rectifying circuit 500. Due to the deployment of an analogue modulation method to modulate the pulse width, this type of converter must incorporate analogue circuits including the current monitoring circuit 512 and the differential amplifier 524. With these analogue devices, the converter has a rather complicated circuit layout and is very sensitive to any process variation. Furthermore, the output voltage may deviate slightly from the target voltage.
FIG. 7 is a circuit diagram of a conventional clock pulse frequency modulation (PFM) direct voltage/direct voltage converter. The clock PFM direct voltage/direct voltage converter comprises a switch-rectifying circuit 500, a first resistor 514, a comparator 722, an AND gate 718 and a square wave generator 720. As shown in FIG. 7, the switch-rectifying circuit 500 has a switch for receiving an input voltage. Thereafter, the input voltage is shaped and filtered to produce an output voltage. The comparator 722 compares the output voltage with a reference voltage and outputs a signal to the AND gate 718. According to the signal received from the square wave generator 720 and the comparator 722, the AND gate 718 outputs a turning-on or a switching-off signal to a switching device 510 inside the switch-rectifying circuit 500. Due to the setup of a fix pulse width in this type of converter, pulse width cannot be modulated according to the actual loading and hence the output voltage contains ripple. Moreover, the converter has little adaptive and feedback mechanisms as the loading changes. Thus, the converter is rather unsuitable to operate in an environment where loading changes are large. In addition, this type of converter is highly sensitive to slight variation in the inductance of any internal inductor. In other words, high cost inductors with very small variation in inductance value must be used.
FIG. 8 is a circuit diagram of a conventional current-limiting pulse frequency modulation (PFM) direct voltage/direct voltage converter. The current-limiting PFM direct voltage/direct voltage converter comprises a switch-rectifying circuit 500, a current monitoring circuit 512, a first resistor 514, a second resistor 516, a first comparator 822, a second comparator 820 and a S-R flip-flop 618. As shown in FIG. 8, the switch-rectifying circuit 500 has a switch for receiving an input voltage. Thereafter, the input voltage is shaped and filtered to produce an output voltage. The first comparator 822 compares the output voltage with a reference voltage and outputs a first signal to the S-R flip-flop 618. The second comparator 820 compares the voltage received from the current monitoring circuit 512 with a second reference voltage to output a second signal to the S-R flip-flop 618. According to the first signal and the second signal, the S-R flip-flop 618 outputs a turning-on or a switching-off signal to a switching device 510 inside the switch-rectifying circuit 500. Due to the deployment of a current monitoring circuit, the circuit of this type of converter is rather complicated and expensive to produce. Moreover, the current monitoring circuit tends to increase power consumption and hence this type of converter is unsuitable for driving a load with a large loading variation.